point of rotation on edge rail post

[SLTA]

ok folks,

I'm checking a platform post attachment. The steel post is attached to (2) 2x12 as shown in the section attached. (Ignore the brackets at top and bottom of the post - they're for a separate location.) I'm checking the withdrawal capacity of the 2 lag screws and am fine when the req'd 200 lbs moveable load is applied horizontally to the right. However, when the load is applied to the left, where would my rotation point be? I've talked myself into the bottom of the post (almost at the bottom of the 2x12s, the center of the two lag screws, and at the top lag screw, and can't decide which is the right one.

Where would you put the rotation point and why?

thanks,
SLTA

 

[Ron]

The bottom of the post is the actual point of rotation. Different loads will be applied to each of the bolts and you could analyze yourself into a stupor if you try to figure out all of the strain mechanisms....so simplify....consider a single resistance between the bolts and then proportion the load between the bolts based on their respective distances from the point of rotation.

 

[SLTA]

AH! That's exactly what I was doing. So glad you're on here late on a Sunday, Ron! cheers!

 

[Ron]

Great minds think alike!!
Was sitting at a traffic light tonight and a van full of "Jacksonville Roller Girls" pulled up. Thought about you.

Cheers!

 

[racookpe1978]

Yeah - Bottom of the post, (about) 2 inches below the lower lag bolt.

(Watch out for the lag bolts coming loose over time in the wood as the wood dries and the posts get vibrated. Could you use through bolts of the same diameter?)

(Also, the lower end of the 2x12 will squish a little bit - which would reduce that 2" inches to maybe 1-1/2 to 1-1/4 effective length, but as it squishes, it resists force more ... but I don't think either effect will be very much.)

 

[BAretired]

Interesting problem but not enough information. If the post bears tight against the deck, then the middle of deck becomes the fulcrum point, i.e. 5.4 inches above the upper bolt irrespective of direction of rail lateral force. If not, then fulcrum point is top of 2x12 for rail force right and bottom of 2x12 for rail force left.

BA

 

[racookpe1978]

BA - So you'd assume the post would "twist" the two 2x12's from under the deck? (Pushing left at the top of the post.)

 

[BAretired]

Pushing left at the rail in the absence of the 2x12's would cause the post to rotate counterclockwise with fulcrum point at deck level and the bottom of post moving to the right.

If the 2x12's are in place, they would tend to rotate counterclockwise under pressure from the post. If they were fixed in place so that they could not rotate, the post would bend as a cantilever fixed at deck level. Provided the deck was capable of taking the post reaction, the lag bolts would feel no load.

BA

 

[racookpe1978]

So the original question will be affected by the configuration of the joists in the deck: Are they perpendicular to the double 2x12 and so will act to prevent rotation, or running parallel to the double 2x12?

 

[BAretired]

Even if the joists are perpendicular to the edge beam they will need a special connection to prevent its rotation.

BA